1. Field of the Invention
The present invention relates to the chemical decontamination of contaminants in soil and groundwater in in-situ and ex-situ environments.
2. Description of the Prior Art
Conventional in-situ treatment technologies for cleaning contaminated subsurface media use injection ports or a combination of injection and extraction ports to deliver reagents and to extract reaction byproducts and contaminants. In-situ chemical oxidation or reduction requires the delivery of reagents in an aqueous medium. Following gravitation, the aqueous reagent solution administered to the subsurface through fixed injection ports becomes an integral part of the groundwater. The volume of contaminated subsurface media in the unsaturated zone above the groundwater table that is affected by the reagent solution is limited to the annular space of the injection ports. Within the groundwater, the reagent solution follows the natural or induced hydraulic gradient. The oxidizing and nucleophilic reagent solution follows preferred pathways, due to physical and chemical heterogeneities of subsurface media. Physical heterogeneities include variability in hydraulic conductivity caused by material changes—for example, clay versus sand versus gravel soils versus fractured bedrock. Mineral surfaces are hydrophilic. The hydrophilic properties are altered by sorption of organic compounds such as natural soil organic matter and organic contaminants that contain both hydrophilic and hydrophobic moieties.
The physical limitations of conventional in-situ delivery systems and the physical chemical heterogeneities of subsurface media limit the effectiveness of oxidizing reagent solutions in making contact with contaminants. Moreover, the oxidizing reagents that are typically utilized in in-situ chemical oxidation systems, e.g. liquid hydrogen peroxide, sodium or potassium permanganate, sodium percarbonate, sodium persulfate, etc., are unstable and/or short-lived with the monovalent sodium or potassium cations negatively affecting site soil by causing soils to become sodic.
Consumption of oxidant by matrix constituents typically exceeds the oxidant consumption by contaminants. To overcome these limitations, large volumes of highly concentrated reagent solutions are typically administered to the contaminated subsurface media. The introduction of highly concentrated and reactive solutions that contain non-specific oxidizing and/or reducing agents poses problems with respect to controlling the progress and the heat of these reactions.
In-situ oxidation systems are known that chemically oxidize organic contaminants to environmentally safe and non-toxic constituents. One such system is a reaction named after its discoverer, H. J. H. Fenton (1894). In this reaction, the oxidizing agent, hydrogen peroxide, is reacted with a metallic salt to generate free radicals with a higher oxidation potential than hydrogen peroxide. The free radicals react with organic compounds to either completely decompose them to carbon dioxide and water or to convert them to water soluble and biologically degradable compounds. A drawback to this process is that the catalytic decomposition of hydrogen peroxide and oxidation of organic compounds by radicals are both exothermic reactions.
A number of patents teach the art of treating contaminants with Fenton-type chemical systems in in-situ environments. The patents by Brown et al., U.S. Pat. No. 4,591,443, Vigneri, U.S. Pat. No. 5,520,483, Wilson, U.S. Pat. No. 5,611,642, Kelly et al., U.S. Pat. No. 5,610,065, and Cooper et al., U.S. Pat. No. 5,967,230, teach the introduction of liquid hydrogen peroxide and a metal catalyst, Fenton's Reagent, such as an iron salt, into the subsurface. Watts et al., U.S. Pat. No. 5,741,427, teaches the injection of a chelated metal catalyst for use in an in-situ chemical oxidation. All of the above cited art adds a metal catalyst into the subsurface. In addition, the processes described in the above cited art include either the co-injection or the sequential introduction of reagents, where the oxidizing agent is added either before or after the metal catalyst. Finally, all of the prior art teaches the necessity of introducing both the oxidizer and the metal catalyst together or separately into the subsurface to facilitate the oxidation of contaminants.
Conventionally, a solution does not exist whereby the use of metal catalyzed peroxides to oxidize underground contaminants is simplified, more controllable, and produces superior results without large amounts of exothermic heat being generated.
It should also be pointed out that the majority of sites are contaminated with multiple types of contaminants. Organic contaminants generally fall into several categories. These include contaminants composed of hydrogen and carbon atoms and are generally referred to as hydrocarbons. A second large cross section of contaminants are composed of hydrogen, carbon and halogen atoms and are known as halogenated compounds. This latter group of compounds is generally more recalcitrant than hydrocarbons.
The most popular methods of remediating halogenated compounds are the application of sodium or potassium permanganate, sodium persulfate, anaerobic reductive dechlorination and the application of nanoscale iron. While popular, these methods have serious complications that make them risky and generally require a long period of time if they are successful at all. Biological reductive dechlorination is dependent upon in-situ factors that will allow microbial proliferation. One of the most serious drawbacks to this technique is that it will not proceed where the concentrations of contaminants are in excess of the toxic threshold of the microbial community. Thus, it is not applicable to high concentrations of contaminants or conditions where free phase product is present. Similarly, although the application of nanoscale iron is not dependent upon biological factors, it is a solid suspension and thus, extremely difficult if not impossible to inject in heavy soils such as hard clay. Therefore, the most popular method of application is trenching, which is expensive and requires the employment of heavy equipment and opening the soil matrix, thereby exposing the contaminant to volatilization to the atmosphere. This practice can produce conditions unsafe for inhalation by site workers. Permanganate salts will successfully mitigate halogenated contaminants, but halogenated compounds are almost always co-contaminants of hydrocarbon compounds that cause permanganate to precipitate as manganese dioxide, thereby causing cessation of the oxidation reaction.
Disposing of produced water or brine by surface discharging was once a common practice by U.S. oil and natural gas producers. This has resulted in extensive damage to the environment in the form of brine scars which are incapable of supporting plant life. Remediation of a brine scar typically involves the removal of salt from the surface layers of soil. However, salt located in deeper layers is rarely removed and can migrate vertically to the surface via capillary action, resulting in the re-contamination of a site and negating remediation efforts. Soil cores taken from a number of natural soil pedestals within a brine-contaminated site have revealed that most soils are co-contaminated at most layers with salt (produced brine water) and organics in the form of hydrocarbons.
Where soils are contaminated with brine or salt to the extent that they do not allow plant growth because of high concentrations of sodium, they are referred to a “sodic.” Such soils are unsuitable landscaping or site restoration due to two adverse properties. These are:
1) salinity, often expressed in terms of the soil's electrical conductivity (EC), and
2) sodicity, often expressed in terms of the soil's sodium adsorption ratio (SAR).
A significant percentage of oil- or gas-producing sites are adversely affected by salinity and sodicity due to sodium contamination from produced water (salt water extracted with oil or natural gas production). Salinity (high EC) directly affects plant growth by hindering or preventing root uptake of water which must occur against an osmotic pressure gradient. The greater the concentration of dissociated, ionized salts in a soil's pore water, the greater the water's charge-carrying capacity and hence the higher the soil's EC. EC is expressed in units such as deciSiemens per meter (dS/m). Below EC=2 dS/m, soils are considered non-saline, and few plant species are affected, but at salinity levels above 12 dS/m, most plant species cannot grow.
Sodicity (high SAR) can cause soil plasticity, leading to difficulties in soil cultivation and to slow rates of water infiltration and drainage. These effects occur with sodic soils containing much clay, and in soils with naturally-occurring sodic subsoils such as solonietzic soil. SAR values of non-sodic soils are usually less than 1 SAR unit. Sodicity problems typically arise when SAR values exceed 10 units, depending on clay content. The SAR is a measure of the ratio of sodium [Na+1] ions (positively charged cations) in the pore water compared to that of calcium [Ca+2] and magnesium [Mg+2] cations. SAR value is calculated using the equation:
  SAR  =            Na              +        1                                                  Ca                          +              2                                +                      Mg                          +              2                                      2            Cation conc. expressed as [Na+1], [Ca+2], [Mg+2]
In the above equation, [Na+1] etc. are cation concentrations in a filtrate of a saturated soil paste. Sodium cations are monovalent (carrying a single positive charge) whereas calcium and magnesium cations are divalent (having two positive charges). In sodic soils, the SAR is correlated with the percentage of cation exchange sites, on clay and organic matter, occupied by sodium cations. As a result of these adverse effects, environmental guidelines are in place regulating permitted levels of EC and SAR in soil and subsoil matrices.
Conventional remedies for mitigation of sodic soils include applications of calcium and/or magnesium compounds such as calcium sulfate (gypsum), calcium nitrate, calcium chloride, and magnesium sulfate (Epsom salts) which dissipate in soil pore water to yield calcium and magnesium cations that are dissociated in solution from attendant negatively charged anions. Alternatively, acids (including both mineral and organic acid) have been applied which reacts with calcium or magnesium carbonates (present in alkaline soil) to release calcium and magnesium cations in-situ. The calcium and magnesium compounds are applied to increase the concentration of calcium and magnesium cations in the soil's pore water thus, restoring the ability of the soil (particularly clay soils) to transport water. As the water permeates the soil, it flushes out sodium ions, thereby lowering the soil's SAR value.
Traditionally, the application of calcium and/or magnesium compounds have been applied to the surface of brine affected soils with some mechanical mixing to incorporate the admixture into shallow soils. Although this method proved somewhat successful for agricultural operations, it was inappropriate for application under oil and natural gas production conditions where brine (produced water) was co-mingled with fuel components at much deeper levels. Also, the carbonate and sulfate salts of calcium and magnesium had no effect toward the remediation of the fuel components. The addition of chelating agents to disassociate calcium and magnesium carbonates and bring the cations into solution has been attempted; however, the practice has no effect on hydrocarbon contaminants, is expensive and has proven less than marginally successful under the broad spectrum of field applications.
Thus, a conventional process does not exist that remediates both the organic contaminants and the sodic (SAR) conditions with the application of a single solution that is applied through the use of methods sufficient to contact these co-contaminants located at deeper as well as shallow levels.